Electric Fields Affect Molecular Twist Within Light-Sensitive Proteins

A team of scientists from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University has gained insight into how electric fields affect the way energy from light drives molecular motion and transformation in a protein commonly used in biological imaging. A better understanding of this phenomenon, which is crucial to many biological systems, could enable researchers to finely tune a system's properties to harness these effects, for example, by using light to control neurons in the brain.

When light hits certain chromophores in proteins, it causes them to twist and change shape. This atomic reconfiguration, known as photoisomerization, changes the molecule's chemical and physical properties. The hallmark of this process is a rotation that occurs around a chemical bond in the molecule. New research shows that the electric fields within a protein play a large role in determining which bond this rotation occurs around. Courtesy of Stanford University.

To find out more about the process, the researchers looked at green fluorescent protein, which is frequently used in biological imaging. Its chromophore can respond to light in a number of ways that are sensitive to its local environment within the protein, producing fluorescent light of various colors and intensities.

Stanford graduate students Matthew Romei and Chi-Yun Lin first tuned the electronic properties of the chromophore within the protein by introducing chemical groups that systematically added or subtracted electrons from the chromophore in order to engineer an electric field effect. They then measured how this affected the chromophore’s twisting motion.

Together with Irimpan Mathews, a scientist at SLAC’s Standard Synchrotron Radiation Lightsource (SSRL), the researchers used an x-ray technique called macromolecular crystallography at SSRL beamlines 7-1, 12-2, and 14-1 to map the structures of these tuned proteins, thereby showing that these changes had little effect on the atomic structure of the chromophore and the surrounding protein. They then measured how changes to the chromophore’s electron distribution affected where rotation occurred when it was hit by light.

“Until now, most of the research on photoisomerization in this particular protein has been either theoretical or focused on the steric effect,” Romei said. “This research is one of the first to investigate the phenomenon experimentally and show the importance of the electrostatic effect. Once we plotted the data, we saw these really nice trends that suggest that tuning the chromophore’s electronic properties has a huge impact on its bond isomerization properties.”

These results also suggest ways to design light-sensitive proteins by manipulating the environment around the chromophore. Lin added that this same experimental approach could be used to study and control the electrostatic effect in many other systems.

“We’re trying to figure out the principle that controls this process,” Lin said. “Using what we learn, we hope to apply these concepts to develop better tools in fields such as optogenetics, where you can selectively manipulate nerves to lead to certain functions in the brain.”

The research was published in Science (www.doi.org/10.1126/science.aax1898).